Screening machines have been used in various industries including the mining and oil industries for many years to enhance the separation of solids and liquids. Within these industries, drilling and mineral extraction processes often produce slurries of solids and liquids that must be separated from one another. As is well known, a screening machine typically includes a screen bed over which a solution containing fluids and solids is passed and then subjected to various separation forces including gravity and shaking. Each screen separation apparatus will utilize different types and sizes of screens to enable separation of different fluids/solids. In addition, the use of vacuum systems to improve separation within screening systems has also been implemented including the use of pulsed vacuum pressure as described in the inventor's co-pending and issued patent applications.
Depending on the industry, the fluid/solid solutions being screened and the commercial objectives of the screening systems, different designs of screening machines exist. In different machines, certain functions have been incorporated into each machine for use within a specific industry or with specific solid/liquid solutions. The nuances of each general type of solid/liquid solution and each machine generally means that one type of machine will not be operative or effective within a different industry as, in many cases, unique problems exist in the handling of specific types of materials or solutions. For example, many screening machine designs have been designed to optimize recovery of the solid materials from within a slurry; however, this format tends to ignore the quality of the recovered fluid. As such, it has generally not been considered how to effect separation of solids and liquids while maintaining or improving the quality of the fluid being recovered.
In the specific case of separating drilling fluid from drill cuttings at a well site, vacuum systems for the separation of drilling fluid from drill cuttings have been effectively deployed in the field in recent years by the applicant. As described in the inventor's co-pending applications and incorporated herein by reference (PCT/CA2009/001555 filed Oct. 29, 2009, PCT/CA2010/00501 filed Mar. 31, 2010, and PCT/CA2011/000542 filed May 11, 2011) the use of a vacuum force on a shaker system, when applied correctly, can be highly effective in reducing drilling fluid retained on cuttings for increasing the quantity of recovered drilling fluid, while also minimizing damage to drill cuttings which can result in contamination of the drilling fluid with fine solid materials that can pass through the screens for increasing the quality of recovered drilling fluid.
Furthermore, the efficiency of shaker systems is important to minimize the costs of solids control processing at a well. For example, at most drilling rigs, multiple shaker systems are installed to simultaneously process drill cuttings from the rig. As is common practice, typically two or more shakers (often 3 or more and potentially up to 9 shakers) are configured to the drilling rig adjacent the blowout preventer (BOP). As drilling fluids and drill cuttings exit the well head, they are conveyed to the shakers via conduits to the possum belly of each the shakers. The conveyed cuttings and drilling fluids are generally split into separate flow streams at the well head in order that a relatively consistent amount of cuttings/fluid is delivered to each shaker.
As can be appreciated, the total number of shakers that may be utilized at a drill site will significantly influence the total costs of the solids handling program. That is, to the extent that fewer shakers are required, the costs of solids handling can be reduced.
In addition, in a typical scenario, shaker systems may be configured in series to one another wherein an upstream shaker may utilize a coarse screen and a downstream shaker may utilize a finer screen. As is understood, the coarse screen will enable relatively finer solids and drilling fluid to pass through the screen and a finer screen will allow drilling fluid to pass through the screen while retaining the finer solids on the upper surface of the screen.
Generally, a balance must be maintained between the pore size of the screen and the desired processing rate. For example, in order to maintain an effective flow rate over a shaker, a combination of coarse and fine screens is usually used such that sufficient volumes of fluid are recovered within a particular time period. That is, if too fine of a screen is used, the time required to process a volume of drill cuttings and drill fluid becomes inefficient, and/or separation of drill cuttings and drill fluid may be prevented due to screen clogging and/or blinding. However, if too coarse a screen is used, the fluid/solids separation becomes inefficient in that the quality of recovered drilling fluid is reduced by solid contaminants.
In the past, various screens and shaker systems have been designed to improve the separation efficiencies including 3-dimensional screen designs and shaker systems. For example, U.S. Pat. No. 6,032,806 describes a “pyramid” style shaker screen in which a three-dimensional screen is used to increase the surface area of the screen. In other systems, shaker systems have been designed to include separate decks for separating solids at different vertical positions within a shaker. However, these past systems remain inefficient in a number of aspects. For example, double deck shakers are more expensive to build in that separate deck and attachment systems, such as clamps, wedges or hooks, are required for each level of deck. In addition, these systems are often significantly taller than a conventional single level shaker.
There are several different attachment systems that are commonly used to secure a screen system to a shaker, specifically within the shaker basket. One such attachment system is a wedge system. The wedge system typically comprises compressing wedges that are located on the sides of the shaker basket, each wedge being driven into a guide located above the position where the screen is located to secure the screen in place in the basket. A compressing wedge is typically about 1 inch wide and 12-18 inches long, and two wedges are typically used per screen.
An alternative attachment system is a plate clamping system, which generally comprises plates or rails located on the sides of the shaker basket that are squeezed together using air or hydraulic pressure to clamp the edge of the screen between the plates/rails. The plates or rails are typically about 1 to 1½″ wide.
A third type of attachment system is a hook screen system that pulls the edges of the screen toward the sides of the shaker basket to apply tension to the screen. This is generally done by using a lever that can attach to the side of the screen with a hook. A force is applied to the lever, pulling the lever through a hole located in the side of the shaker basket, thereby pulling the screen outwards to apply tension to the screen. Typically the force applied to the lever is a spring force, however in some designs the spring is replaced with a bolt and screw arrangement which is adjusted to a predetermined torque, or with an air or hydraulic piston assembly. With the hook screen attachment system, the screen may be pulled over a flat surface or a curved surface, such as a convex surface. Pyramidal style shaker screens are often attached to shakers using a hook screen attachment. An example of a hook screen attachment system is described in U.S. Pat. No. 6,179,128.
A problem with prior art shakers is the effect of both large and small particles on a screen. That is, larger particles have the tendency to impact a screen with greater force due to the momentum of the particle. Fine screens, with narrower and less strong wires may be degraded more rapidly as a result of impact with larger particles. Thus, a layered screen system with a coarser upper screen and a finer lower screen has the advantage of protecting the lower screen from larger and potentially damaging particles as these particles will be carried on the upper screen and will not transit through the coarse screen to impact the fine screen below.
Another issue is that it is important to ensure that a layered screen system won't be compromised by the flow of drill cuttings and drilling fluid over the shaker such that the performance of the screens/shaker is affected. In particular, it is important that the gap between a lower screen and upper screen does not become clogged if the flow of drilling fluid/drill cuttings through the gap becomes high due to the volume of material in the shaker.
As a result, there continues to be a need for systems that improve the effectiveness of shaker systems to enable the sequential separation of coarser and fine solids. In addition, there is also a need for systems that can be retrofit to existing shaker systems, including existing shaker system attachment systems, to effectively turn single deck systems into double-deck screening systems.